Various seismic techniques have been developed which employ a seismic receiver array disposed in a borehole in an earth formation. Examples include tomographic techniques (such as include cross-borehole seismic tomography), and Vertical Seismic Profiling (VSP).
Distributed Acoustic Sensing (DAS) is a useful novel technology to provide such a seismic receiver array in a borehole in an earth formation for seismic data acquisition purposes. A description of this technology is provided in an article “Distributed acoustic sensing for reservoir monitoring with vertical seismic profiling” by Albena Mateeva et al., which appeared in Geophysical Prospecting, Vol. 62, pp. 679-692 (2014). Conceptually, DAS measurements are simple. A DAS interrogator unit sends laser pulses along an optical fiber disposed in a wellbore, and measures signals of back-scattered light. The optical fiber can be subdivided into DAS receiver channels (corresponding, for instance, to VSP receiver levels) based on the time of flight of a laser pulse along it. However, pinpointing the exact physical depth of a DAS receiver channel with respect to geology, is not trivial, and requires some calibration of the optical depths versus depths in the borehole.
Similarly, there can be a need for depth calibration of seismic receiver arrays of other types of seismic receiver channels, such as arrays of geophones.
A method for determining the location of a fiber optic channel is described in US 2013/0279841. The location of one or more fiber optic channels in this method is determined by:
a) arranging an electrical conductor and a magnetic source at a known location adjacent to at least one of the channels;
b) transmitting an electrical current through the electrical conductor, thereby deforming the electrical conductor by Lorenz forces in the vicinity of the magnetic source;
c) conveying the deformation of the electrical conductor to deform an adjacent channel;
d) transmitting light pulses through the fiber optic cable and using variations in the light pulses back reflected by the deformed channel and the known location of the magnetic source to determine the location of the deformed channel.
A drawback of this known method for determining the location of a fiber optic channel is that it requires additional equipment in the wellbore in order to locally deform the optical fiber at a known depth in order to be able to use the variations in the light pulses back reflected by the deformed channel and the known depth to determine the depth of the deformed channel relative to an external frame of reference (such as a geology).